Whine or whistle from 3.9L

This van I bought has a high pitched whine or whistle when driving around town. It occurs when you step on the gas. The faster the engine turns the more high pitch it becomes. When the foot comes off the gas it becomes louder but lower pitched for a second or two. At idle the noise isn't noticed and isn't heard at highway speeds. I'd like to know whether it's a noisy alternator or a vacuum leak? I was assuming it was the alternator but then it could be associated with the check engine light that came on once recently, showing code 0121.